P(νμ→νe) = sin²(2θ)·sin²(Δm²L/4E) — quantum flavor mixing
Neutrinos are produced as flavor eigenstates (νe, νμ, ντ) but propagate as mass eigenstates (ν₁, ν₂, ν₃). The mismatch causes oscillations with probability P(νμ→νe) = sin²(2θ)·sin²(1.27·Δm²·L/E). The MSW (Mikheyev-Smirnov-Wolfenstein) matter effect modifies the effective mixing when neutrinos traverse dense matter, explaining the solar neutrino problem. Δm²₁₂ ≈ 7.4×10⁻⁵ eV², θ₁₂ ≈ 33.5°.